Ion chromatography systems with flow-delay eluent recycle

ABSTRACT

A chromatographic method including chromatographically separating sample ionic species in an eluent stream, detecting the separated sample ionic species, catalytically combining hydrogen and oxygen gases or catalytically decomposing hydrogen peroxide in a catalytic gas elimination chamber, and recycling the effluent stream from the chamber to the chromatography separation column. The residence time between the detector and the chamber is at least about one minute. Also, flowing the recycle sequentially through two detector effluent flow channels of an electrolytic membrane suppressor. Also, applying heat or UV energy between the detector and the chamber. Also, detecting bubbles after the chamber. Also, a Platinum group metal catalyst and ion exchange medium in the chamber. Apparatus for performing the methods.

BACKGROUND OF THE INVENTION

Since it was introduced in 1975, ion chromatography has become a widelyused analytical technique for the determination of anionic and cationicanalytes in various sample matrices. In ion chromatography, dilutesolutions of acids, bases, or salts are commonly used as theelectrolytes in chromatographic eluents.

Traditionally, these eluents are prepared off-line by dilution withreagent-grade chemicals. Off-line preparation of chromatographic eluentscan be tedious and prone to operator errors, and often introducescontaminants. For example, dilute NaOH solutions, widely used as theelectrolytes in eluents in the ion chromatographic separation of anions,are easily contaminated by carbonate. The preparation of carbonate-freeNaOH eluents is difficult because carbonate can be introduced as animpurity from the reagents or by adsorption of carbon dioxide from air.The presence of carbonate in NaOH eluents often compromises theperformance of an ion chromatographic method, and can cause anundesirable chromatographic baseline drift during the hydroxide gradientand even irreproducible retention times of target analytes. Therefore,there is a general need for convenient sources of high purity acid,base, or salt for use as eluents in the ion chromatographic separations.

The continuous operation of an ion chromatography system can consume asignificant amount of eluents. The consistent preparation of such largeamount of the eluent as well as the disposal of the used eluent can poseserious logistical challenges to the system operators in terms of costsand labor, especially in cases where unattended or less frequentlyattended operations are required. Even though it overcomes a number ofissues associated conventional approaches of eluent preparation in ionchromatography, the use of on-line electrolytic eluent generationdevices still requires a constant supply of high purity water from anexternal source for continuous operation and waste disposal issueremains.

U.S. Pat. No. 7,329,346 describes ion chromatography systems capable ofrecycling eluents. In one embodiment, the electrolytic suppressor isoperated in the recycle mode. The net result of the electrochemicalprocesses in an electrolytic suppressor is that the combined effluentfrom the suppressor anode and cathode chambers is a mixture of hydrogengas, oxygen gas, and the aqueous solution containing the eluentcomponents, the ions from the sample injected, and possibly some tracecomponents derived from the operations of the separation column andsuppressor. The effluent from the outlet of the electrolytic suppressorregenerant chamber is passed through the catalytic gas eliminationcolumn packed with a Pt catalyst that induces the reaction betweenhydrogen gas and oxygen gas to form water. The catalytic gas eliminationcolumn serves several important functions. First, it provides an elegantmeans to conveniently eliminate the build up of hydrogen and oxygengases and thus facilitates the operation of continuous eluent recycle.Second, the water-forming reaction of hydrogen and oxygen isstoichiometric in the column, and the amount of water formed is expectedto be in principle the same as the amount of water consumed originallyto produce hydrogen and oxygen gases in the electrolytic operation ofthe suppressor. In the above embodiment, an analyte trap column isplaced after the outlet of a conductivity detector to trap analyte ions.Additionally, ion exchange eluent purification columns packed withappropriate ion exchange resins are used to further purify theregenerated eluent for use for re-use as the ion chromatographic eluentin the separation process.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a chromatographic method is providedincluding the steps of (a) injecting sample ionic species into anaqueous eluent stream from an eluent source, (b) chromatographicallyseparating the sample ionic species in the eluent stream by flowing thesame through chromatographic separation medium to exit as achromatography effluent, (c) flowing the chromatography effluent througha detector to detect the separated sample ionic species in thechromatography effluent to exit as a detector effluent stream, (d)catalytically combining hydrogen and oxygen gases or catalyticallydecomposing hydrogen peroxide, or both, in the detector effluent streamby flowing it through a catalytic gas elimination chamber, to form waterand reduce the gas content of the eluent effluent stream exiting the gaselimination chamber, and (e) recycling the catalytic gas eliminationchamber effluent stream from the catalytic gas elimination chamber tothe chromatography separation column, the residence time for flow of thedetector effluent stream between the detector and the catalytic gaselimination chamber being at least about one minute to facilitatedecomposition of unstable oxidative compounds.

In another embodiment, a chromatographic method is provided comprisingthe steps of (a) chromatographically separating sample ionic species inan aqueous liquid eluent stream flowing through a chromatographyseparation medium, to form a chromatography effluent, (b) suppressingthe chromatography effluent from step (a) by flowing it through achromatography effluent flow channel in an electrolytic membranesuppressor to exit as a suppressor effluent stream, (c) flowing thesuppressor effluent stream past a flow-through detector to exit as adetector effluent stream, (d) flowing the detector effluent stream fromstep (c) through a first detector effluent flow channel in the membranesuppressor on the opposite side of a first ion exchange membrane fromthe chromatography effluent flow channel, the detector effluent exitingthe first detector effluent flow channel as a recycle stream, (e)flowing the recycle stream through a catalytic gas elimination chamberto catalytically combine hydrogen and oxygen gas, or catalyticallydecomposing hydrogen peroxide, or both, to form water thereby reducingthe gas content in said recycle stream, and (f) flowing the recyclestream from the catalytic gas elimination chamber to the chromatographyseparation medium, the time for flow of the detector effluent from thefirst detector effluent flow channel to the catalytic gas eliminationchamber being at least one minute to facilitate decomposition ofunstable oxidation compounds.

In another embodiment, a chromatographic method is provided comprisingthe steps of (a) injecting sample ionic species into an aqueous eluentstream from an eluent source, (b) chromatographically separating sampleionic species in an aqueous liquid eluent stream flowing through achromatography separation medium, to form a chromatography effluent, (c)suppressing the chromatography effluent from step (a) by flowing itthrough a chromatography effluent flow channel in an electrolyticmembrane suppressor to exit as a suppressor effluent stream, (d) flowingthe suppressor effluent stream past a flow-through detector to exit as adetector effluent stream, (e) flowing the detector effluent stream fromstep (c) through a first detector effluent flow channel in the membranesuppressor on the opposite side of a first ion exchange membrane fromthe chromatography effluent flow channel, and then through a seconddetector effluent flow channel on the opposite side of a second ionexchange membrane in the membrane suppressor from the chromatographiceffluent flow channel, the detector effluent exiting the second detectoreffluent flow channel as a recycle stream, (f) flowing the recyclestream through a catalytic gas elimination chamber to catalyticallycombine hydrogen and oxygen gas, or catalytically decomposing hydrogenperoxide, or both, to form water thereby reducing the gas content insaid recycle stream, and (g) flowing the recycle stream from thecatalytic gas elimination chamber to the chromatography separationmedium.

In another embodiment, a chromatographic method is provided comprisingthe steps of (a) injecting sample ionic species into an aqueous eluentstream, (b) chromatographically separating the sample ionic species inthe eluent stream by flowing the same through chromatographic separationmedium, (c) detecting the separated sample ionic species in the eluentstream effluent from said chromatographic medium, (d) catalyticallycombining hydrogen and oxygen gases or catalytically decomposinghydrogen peroxide, or both, in the eluent stream in a catalytic gaselimination chamber, to form water and reduce the gas content in aneluent stream, in an effluent stream flowing from said catalytic gaselimination chamber, and (e) detecting bubbles in the catalytic gaselimination chamber effluent stream.

In another embodiment, a chromatographic method is provided comprisingthe steps of (a) injecting sample ionic species into an aqueous eluentstream, (b) chromatographically separating the sample ionic species inthe eluent stream by flowing the same through chromatographic separationmedium, (c) detecting the separated sample ionic species in the eluentstream effluent from the chromatographic medium, (d) catalyticallycombining hydrogen and oxygen gases or catalytically decomposinghydrogen peroxide, or both, in the eluent stream in a catalytic gaselimination chamber, to form water and reduce the gas content in aneluent stream, effluent exiting from the catalytic gas eliminationchamber, and (e) between steps (c) and (d), applying energy to the gaselimination effluent to decompose at least a portion of any unstableoxidative compounds therein.

In another embodiment, a chromatograph apparatus is provided including(a) a chromatography column defining a column lumen, (b) flow-throughchromatographic separation medium disposed in the column lumen, theseparation medium defining liquid flow-through passages, (c) a detector,(d) a conduit for an aqueous liquid eluent stream providing fluidcommunication between the detector and the chromatographic separationmedium, (e) a catalytic gas elimination chamber in fluid communicationwith the conduit and including a catalyst for combining hydrogen andoxygen gases, or for catalytically decomposing hydrogen peroxide, orboth, in the eluent stream to form water and reduce the gas content inthe eluent stream, and (f) a delay conduit disposed between the detectorand the catalytic gas elimination chamber, the delay conduit having atotal flow-through volume of at least 0.5 times the total volume of saidseparation medium flow-through passages.

In another embodiment, a chromatography apparatus is provided including(a) chromatographic separation medium, (b) a detector, (c) a membranesuppressor comprising a chromatography effluent flow channel, first andsecond detector effluent flow channels, and first and second ionexchange membranes separating the chromatography effluent flow channelfrom the first and second detector effluent flow channels, respectively,(d) a first conduit providing fluid communication between the separatormedium and the chromatography effluent flow channel, (e) a secondconduit providing fluid communication between the membrane suppressorchromatography effluent flow channel and the detector, (f) a thirdconduit providing fluid communication between the detector and the firstdetector effluent flow channel, (g) a fourth conduit providing fluidcommunication between the first and second detector effluent flowchannels, (h) a fifth conduit providing fluid communication between thesecond detector effluent flow channel and the separation medium, and (i)a catalytic gas elimination device in fluid communication with at leastone of the first, second or third conduits and including a catalyst forcombining hydrogen and oxygen gases in at least one of the conduits.

In another embodiment, a chromatography apparatus is provided including(a) chromatographic separation medium, (b) a detector, (c) a conduit foran aqueous liquid eluent stream providing fluid communication betweensaid detector and said chromatographic separation medium, (d) acatalytic gas elimination chamber in fluid communication with theconduit and including a catalyst for combining hydrogen and oxygengases, or for catalytically decomposing hydrogen peroxide, or both, inthe eluent stream to form water and reduce the gas content in saideluent stream, and (e) a bubble detector downstream of the catalytic gaselimination chamber and in fluid communication therewith.

In another embodiment, a chromatography apparatus including (a)chromatographic separation medium, (b) a detector, (c) a conduit for anaqueous liquid eluent stream providing fluid communication between thedetector and the chromatographic separation medium, (d) a catalytic gaselimination chamber in fluid communication with the conduit andincluding a catalyst for combining hydrogen and oxygen gases, or forcatalytically decomposing hydrogen peroxide, or both, in the eluentstream to form water and reduce the gas content in the eluent stream,and (e) an energy generator disposed between and in fluid communicationwith the detector and the catalytic gas elimination chamber.

In another embodiment, a catalytic gas and ionic species removal deviceis provided including a liquid flow-through housing, a platinum groupmetal catalyst for catalytically combining hydrogen and oxygen gases, orfor catalytically decomposing hydrogen peroxide, or both, disposed inthe housing, and flow-through ion exchange medium disposed in thehousing.

In another embodiment, a method is provided of catalytically combininghydrogen and oxygen gases, or for catalytically decomposing hydrogenperoxide, or both, and of removing analyte ions, counter-ions, or both,in a flowing liquid sample stream, the method comprising combininghydrogen and oxygen gases or catalytically decomposing hydrogen peroxidein a liquid sample stream containing ions by flowing the liquid samplestream through a flow-through catalytic device containing a platinumgroup metal catalyst capable of catalyzing the combining or decomposing,and removing the ions from the liquid sample stream in the flow-throughdevice by contact with flow-through ion exchange medium disposed in thedevice.

In another embodiment, a chromatographic method is provided includingthe steps of (a) injecting sample ionic species into an aqueous stream,(b) chromatographically separating the sample ionic species in theaqueous stream by flowing the same through chromatographic separationmedium while applying an electric field across the separation medium, toexit as a chromatography effluent, (c) flowing the chromatographyeffluent through a detector to detect the separated sample ionic speciesin the chromatography effluent to exit as a detector effluent stream,(d) catalytically combining hydrogen and oxygen gases or catalyticallydecomposing hydrogen peroxide, or both, in the detector effluent streamby flowing it through a catalytic gas elimination chamber, to form waterand reduce the gas content of the eluent effluent stream exiting the gaselimination chamber, and (e) recycling the catalytic gas eliminationchamber effluent stream from the catalytic gas elimination chamber tothe chromatography separation column.

In another embodiment, a chromatography apparatus is provided including(a) a chromatography column including chromatographic separation mediumdisposed in the column lumen, (b) spaced electrodes in electricalcommunication with the separation medium and disposed to pass anelectric current through the separation medium, (c) a detector, (d) aconduit for an aqueous liquid stream providing fluid communicationbetween the detector and the chromatographic separation medium, and (e)a catalytic gas elimination chamber in fluid communication with theconduit and including a catalyst for combining hydrogen and oxygengases, or for catalytically decomposing hydrogen peroxide, or both, inthe eluent stream to form water and reduce the gas content in the eluentstream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 are schematic representations of apparatus according todifferent embodiments of the present invention.

FIGS. 9-16 are graphs illustrating experimental results using thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The system of the present invention is useful for determining ionicspecies which are solely anions or cations. Suitable liquid samplesinclude surface waters, other liquids such as industrial chemical waste,body fluids, beverages or drinking water. The term “ionic species”includes molecular species in ionic form and molecules which areionizable under the conditions of the present invention. The term“eluent” refers to the solution flowing in a liquid chromatographysystem which carries a sample to be detected. At times herein, the termeluent also refers to the electrolyte in that solution. The eluentnormally is water-based but can include an organic solvent so long as itis electrochemically stable.

In certain embodiments, the invention includes a suppressor. The purposeof a suppressor is to reduce the conductivity and noise of the analysisstream background while enhancing the conductivity of the analytes(i.e., increasing the signals/noise ratio), while maintainingchromatographic efficiency.

In other embodiments, the invention relates to improved catalytic gasremoval devices and methods and to improved systems for using catalyticgas removal devices and systems disclosed in U.S. Pat. No. 7,329,346,incorporated in its entirety by reference. The invention will first bedescribed for the embodiment of FIG. 1.

Referring to FIG. 1, a simplified apparatus for performing the presentinvention is illustrated. This system includes a suppressor with recycleof the effluent from a detector to the suppressor. That portion of thesystem is similar to the general system illustrated in U.S. Pat. No.5,248,426. In the present invention, the effluent from the suppressor isrecycled to the separation column, preferably after mixed with an eluentin an effluent reservoir, as part of the eluent electrolyte used forseparation. In a preferred embodiment, one or more eluent purificationcolumns such as ion trap columns are used in the system to removecontaminants, typically in ionic form, in the recycled stream prior touse as an eluent in the separation column.

Referring again to FIG. 1, the system includes chromatographicseparation medium, typically in the form of chromatographic separationmedium chromatographic column 10. (As used herein, the term “column”refers to a flow-through housing with an interior chamber in anyconfiguration for performing the indicated function). Any knownchromatographic separation medium may be employed including ion exchangeresin in a resin bed, monolith, or other form, a porous hydrophobicchromatographic resin permanently attached to ion exchange sites, andmedium used for mobile phase ion chromatography (MPIC).

Arranged in series with column 10 is a suppressor 12 serving to suppressthe conductivity of the electrolyte of the eluent from column 10 but notthe conductivity of the separated ions. The effluent from suppressor 12is directed to a detector, preferably in the form of a flow-throughconductivity detector cell 14, for detecting the ion species resolved incolumn 10. A suitable sample including ionic species is supplied throughsample injection valve or injector 16 which is passed through theapparatus in the solution of eluent from an eluent source or reservoir18 drawn by pump 20 which then passes through injection valve 16. Thechromatography effluent solution leaving column 10 is directed throughsuppressor 12 wherein the electrolyte is converted to a weaklyconducting form. The chromatography effluent from suppressor 12 passesthrough detector 14, schematically illustrated as a conductivity cell,in which the presence of ionic species produces an electrical signalproportional to the amount of ionic material. Such a signal is typicallydirected from detector 14 to a conductivity meter (not shown). Any otherknown detector useful for detecting ionic species in a chromatographysystem may also be employed including absorbance and electrochemicaldetectors.

In one embodiment, the effluent from conductivity detector 14, referredto as the detector effluent, is directed in a recycle conduit 22 to atleast one flow-through detector effluent flow channel 24 in suppressor12. An ion exchange membrane 26 separates detector effluent flow channel24 from chromatographic separation effluent flow channel 28 whichreceives the effluent from chromatography column 10. In the simplifiedversion illustrated, only a single detector effluent flow channel 24 isused. The system of the present invention is also applicable to othermembrane suppressors such as the sandwich suppressor type illustrated inU.S. Pat. No. 5,248,426. In a sandwich suppressor, the chromatographicseparation effluent flows through a central flow channel flanked by twodetector effluent flow channels separated by ion exchange membranes. Inthis embodiment, the detector effluent flow channels may be suppliedwith the detector effluent from conductivity detector 14 by use of asplitter valve. The details of such a sandwich suppressor and the use ofrecycle from the conductivity cell are supplied by the detector effluentflow channel as illustrated in U.S. Pat. No. 5,248,426. As illustrated,for anion analysis, the detector effluent flow channel is positivelycharged and hydronium ions are generated for passage through membrane 26according to the following equation:

6H₂O→4H₃O+O₂+4e ⁻

For anion analysis, in the chromatography effluent flow channel, cationsof the electrolyte, e.g., sodium ions, pass through membrane 26 into thedetector effluent flow channel 12 toward a cathode, not shown, forelectrolytic suppressor. Hydroxide is converted to water according tothe following equation:

OH⁻+H₃O⁺→2H₂O

In one preferred embodiment, the suppressor is of the electrolytic typeas illustrated in FIGS. 3 and 4 of U.S. Pat. No. 5,352,360.

Suitable eluent solutions for anion ion chromatography include alkalihydroxides, such as sodium hydroxide, alkali carbonates andbicarbonates, such as sodium carbonate, alkali borates, such as sodiumborate, combinations of the above, and the eluent systems of theaforementioned patents.

The recycle system of the present invention is also applicable to theanalysis of cations (e.g., lithium, sodium, ammonium, potassium,magnesium, and calcium). In this instance, the electrolyte of the eluentis typically an acid which does not damage the membrane. Methanesulfonicacid has been found to be inert to the membrane under electrolyticconditions. Other acids such as nitric acid and hydrochloric acidproduce electrochemical by-products that may damage the membrane andare, thus, not generally preferred for that typical membrane.

In the effluent recycle system of U.S. Pat. No. 5,248,426, the effluentfrom the detector effluent flow channel is directed to waste. Incontrast, in the system of FIG. 1, the effluent stream is redirected tothe separation column 10, preferably by flowing through the eluentreservoir 18. The FIG. 1 embodiment is similar to the system of U.S.Pat. No. 7,329,346 incorporated by reference, except for the flow delayprior to the catalytic gas elimination column.

Referring specifically to FIG. 1, the effluent from the detectoreffluent channel flows in line 30 through catalytic gas eliminationcolumn 31, optional delay conduit 46, and optional eluent purificationcolumn 32 and from there through tubing projecting through a closure 36of container 38 of eluent reservoir 18. An optional gas vent 40 isprovided in reservoir container 38 to vent hydrogen and oxygen gaseswhich are generated electrolytically in the system. Eluent solution fromreservoir 18 is directed in line 42 to separation column 10 as thesource of eluent for separation. As illustrated, the eluent in line 42flows through pump 10 and optional eluent purification column 44 priorto separation column 10.

Also as illustrated in FIG. 1, optional ion trap column 23 can be placedin line 22 between conductivity cell 14 and suppressor 12. Preferably,the ion trap column 23 is packed with anion exchange resin in carbonateform for anion analysis using carbonate eluents or anion ion exchangeresin in the hydroxide form for anion analysis using hydroxide eluents.Typically, it only removes ions of one charge, positive or negative. Forcation analysis using acid eluents, the ion trap column may be packedwith cation exchange resin in the hydronium form. The ion trap columnserves the function for retaining analyte ions in the suppressed eluent.

For a system in which eluent is recycled from the detector to theseparation column, it is preferable to remove contaminants of sampleinjected through injection valve 16, and other trace contaminants may begenerated from operation of the ion chromatography system. This can beperformed by using one or both of purification columns 32 or 44. Oneform of eluent purification column preferably includes an inlet sectionand an outlet section, not shown, with a strongly acidic cation exchangematerial, e.g., resin in the inlet section, preferably in the form ofthe cation of the eluent used. For example, the resin preferably in theform of sodium for a sodium carbonate eluent or in the hydronium formfor a sulfuric acid eluent. The outlet section may be packed withstrongly basic cation exchange material, e.g., resin in the form of theanion of the flowing eluent. For example, the form may be carbonate forthe sodium carbonate eluent or sulfate for the sulfuric acid eluent. Itis preferable to use highly cross-linked and macroporous high area forboth the cation exchange resin and the anion exchange resin used inpurification columns. It is preferable to use resins havingcross-linking of at least 20%, preferably at least 30% and surface areaof at least 10 m²/g, preferably at least 20 m²/g. Such resins areeffective in removing components of the injected sample and other tracecontaminants generated in the system. Examples of such resins include AGMP-50 strongly acidic cation exchange resin and AG MP-1M strongly basicanion exchange resin available from Bio-Rad (Hercules, Calif.). Otherforms of optional eluent purifiers may be employed.

Optionally, the eluent purification columns 32 or 44 may also include anadditional section of neutralized porous resin, preferably of highsurface area to remove non-charged contaminants in the recycle eluent.In the illustrated embodiment, a larger eluent purification column 32 isplaced upstream of suppressor 12 in comparison to the optional smallerpurification column 44 placed at the outlet of pump 20 to further purifythe eluent prior to entering separation column 10. Suitably, column 32has an ion exchange capacity of at least 0.5 milliequivalents.

In general, the eluent purification column and the ion trap columnremove the ions from the sample injected and some trace componentsderived at the system. When the purified eluent is recycled back intoeluent reservoir 18, the solution typically contains a mixture ofelectrolytically generated hydrogen gas and oxygen gas. Some of thesegases may be removed from the eluent reservoir 18 equipped through a gasvent port 40.

A catalytic gas elimination column 31 is particularly useful in a systemin which an electrolytic suppressor with detector, effluent recycle tothe suppressor regeneration flow channel, e.g. of the type disclosed inU.S. Pat. No. 5,248,426, is used. Such a system, sold by DionexCorporation under the trademark SRS, can be used for anion analysis,e.g., using a sodium carbonate eluent. In this mode, the effluent fromthe detector cell is used as the source of water for the electrolysisreactions in the anode and cathode chambers of the suppressor. Under theapplied electrical field, water is oxidized to form hydronium ions andoxygen gas at the anode and reduced to hydroxide ions and hydrogen gasat the cathode. The hydronium ions migrate across the cation exchangemembrane into the eluent chamber of the suppressor to react withcarbonate ions in the eluent to form carbonic acid which is only weaklyconductive. In the meantime, the counter ion in the sample injected(e.g., Na⁺) is replaced with hydronium ion and forced to migrate acrossanother cation exchange membrane into the cathode chamber of the device.The analyte anion, X⁻, is detected in the more conductive form of H⁺+X⁻by the conductivity detector. The net result of the electrochemicalprocesses is that the combined effluent from the suppressor anode andcathode chambers is a mixture of hydrogen gas, oxygen gas, and theaqueous solution containing the ionic eluent components, the ions fromthe sample injected, and possibly some trace components derived from theoperations of the separation column and suppressor.

The electrolysis reactions in the anode and cathode chambers of theelectrolytic suppressor may lead to the formation of reactive oroxidative species. For example, ozone may be formed in the anode chamberof the electrolytic suppressor (H₂O→2H⁺+2e⁻+⅓O₃). Hydrogen peroxide maybe formed in the cathode chamber of the electrolytic suppressor(2H₂O+O₂+2e⁻→2OH⁻+H₂O₂). When an anion electrolytic suppressor is usedto suppress sodium carbonate eluents, the electrolytic formation ofsodium percarbonate may also occur in the anode chamber. The presence ofthese unstable reactive or oxidative species in the effluent of anelectrolytic suppressor may have detrimental effects on the performanceof ion chromatography system with eluent recycle.

The unstable reactive or oxidative species such as ozone, hydrogenperoxide, and sodium percarbonate may attack the ion exchange functionalgroups in the eluent purification column and the separation column inthe system and degrade the ion exchange capacity of the columns. If theeluent purification column and the separation column lose their desiredion exchange capacity, the performance of an ion chromatography systemwith eluent recycle is compromised. For example, the gradual loss of theion exchange capacity of the separation column leads to the downwarddrift of retention time of target analytes separated on the separationcolumn. The gradual loss of the ion exchange capacity of eluentpurification columns reduces their capability in removing components ofsamples injected and other trace contaminants that may be generated fromthe operation of the entire ion chromatography with eluent recycle.

Thus, the aqueous recycle stream which flows through catalytic gaselimination column 31 can include unstable oxidative compounds such asozone and sodium percarbonate. Such unstable oxidative compoundstypically have a relatively short half-life time. For example, ozone hada half-life time of about 3 minutes at pH 9.2 and decomposes into oxygenin aqueous solution. Therefore, it is advantageous to provide a timedelay in the system, particularly between the electrolytic suppressorand the catalytic gas elimination column. This would permit the unstableoxidative species that may present in the electrolytic suppressorregenerant channel 12 effluent recycled to catalytic gas eliminationcolumn 31 to decompose into non-reactive or non-oxidative species.Another advantage of the time delay would be to minimize their potentialdetrimental effects on the ion exchange functional groups in eluentpurification column 32 and separation column.

Absent a time delay, such as performed in delay conduit 46, flow of therecycle stream through a system of the type illustrated in FIG. 1between either cell 14 or suppressor 12 and catalytic gas eliminationcolumn 31 would take a relatively short time. According to oneembodiment of the invention, to facilitate decomposition of unstableoxidative compounds, the residence time for flow between the detectorand the catalytic gas elimination column is at least 1 minute,preferably at least 2 minutes, more preferably at least 5 to 20 minutesor more. According to another embodiment, the time for flow of recyclefrom detector effluent flow channel 24 of suppressor 12 and catalyticgas elimination column 31 is at least 0.5 minutes, preferably at least 2minutes, more preferably at least 5 to 20 minutes or more.

As used herein, the term “residence time” encompasses the time foronline flow between the designated devices and also the delay time forstopped flow in a discontinuous system. For example, with appropriatevalving, part of the recycle stream could be sent to a stopped flowchamber and the flow toggled between the stopped flow and onlinecontinuous flow. Thus, the residence time would include the time thatthe recycle solution is in a stopped flow chamber. Preferably, in thissystem, all flow from the detector is sent to the stopped flow chamberfor the desired time.

As used herein, the term “delay conduit” refers to all tubing anddevices through which the recycle stream flows between the detector orthe suppressor and the catalytic gas elimination column, depending onthe context of use of this term regarding the end points of the delayconduit.

In one embodiment, the delay conduit includes a delay housing and thedetector effluent stream flows from the detector (or the suppressor) ina tortuous path, such as a serpentine path, to maximize the residencetime of gas bubbles in the device. One such delay conduit is illustratedin FIG. 2. It includes flow-through housing or column 50, e.g. ofcylindrical shape, with inlet and outlet ports 50 a and 50 b,respectively. As illustrated, disposed in housing 50 are baffles 52 withflow openings 52 a, unaligned in the direction of flow to cause therecycle solution to flow in a non-linear path 54 and thus delay flow.

FIG. 3 shows another embodiment of a flow-through delay column. In thisembodiment, the delay column is packed with segments of multiplethin-wall tubes so that there are multiple pathways for the gas bubblesto move through the delay column to increase their effective residencetime in the delay column. Referring specifically to FIG. 3, column 60includes inlet and outlet 60 a and 60 b, respectively. It includes aplurality of tubes 62, each being in parallel communication with thedetector. The detector effluent flowing into column 60 through inlet 60a is split into multiple streams through tubes 62 and recombined on exitthrough outlet 60 b, thereby delaying the time of flow and increasingthe residence time of the recycle stream and gas bubbles in the column.

In another embodiment, not shown, the delay conduit can include a largevolume device which is open or filled with packing other than thebaffles of FIG. 2 to further delay flow. Such packing can include apacked bed, e.g. of polypropylene beads or a porous polymer monolith.

In another embodiment, not shown, the delay conduit may comprise longtubing. For space considerations, such long tubing would preferably becoiled, e.g. in a spool of inert polymeric tubing of appropriateinternal diameter and length. The tubing may be fitted with appropriateinlet and outlet fittings. It is preferred that such tubing would havean appropriate wall thickness and is substantially impermeable tohydrogen and oxygen gases so that there is no leakage of hydrogen oroxygen gas across the wall of the tubing. If the leakage of hydrogen andoxygen gas occurs, the stoichiometric ratio of hydrogen and oxygen gasesentering the catalytic gas elimination column would be altered and thusthe water-forming reaction of hydrogen and oxygen becomesnon-stoichiometric in the catalytic gas elimination column, leading toincomplete removal of hydrogen and oxygen gas. In an ion chromatographysystem with eluent recycle, the incomplete removal of hydrogen andoxygen gas by the catalytic gas elimination column may result in theaccumulation of hydrogen gas in the eluent reservoir and thus apotential explosion risk. Therefore, it is preferred that there is noleakage of hydrogen or oxygen gas across the wall of the flow-throughdelay tubing.

Another embodiment of the flow-through delay conduit takes a form of achamber as in a chromatographic column housing with flow-through endfittings. The column housing should have appropriate internal diameterand length to provide the desirable delay volume. The effluent from anelectrolytic suppressor is a mixture of H₂, O₂, and O₃ gases, and theaqueous solution containing mainly the ionic eluent components. Becausethe effluent from an electrolytic suppressor is a gas-liquid mixture, itis desirable to maximize the residence time of ozone-containing gasbubbles in such a flow-through delay column. If the delay column ismounted vertically, the ozone-containing gas bubbles may effervescerapidly in an upward direction through the internal chamber of theflow-through delay column so that the effective residence time in thedelay column is reduced. Therefore, it may be preferred to mount thedelay column horizontally so that gas bubbles move at the same rate thatthe liquid flows through the column.

The chromatographic column defines a column lumen in which flow-throughchromatographic medium is disposed. The medium, e.g. a packed bed or amonolith, defines liquid flow-through passages. The total volume of suchpassages can be measured by the volume of liquid retained by the columnwith the medium in place. This will be referred to as the total volumeof the flow-through passages or “the chromatography column void volume.”Similarly, the delay conduit may include various types of packing. Thetotal volume of the delay conduit is defined by the total volume ofliquid which would be retained by the full length of the variouscomponents of the delay conduit in stopped flow.

In one embodiment, the delay conduit flow-through total volumepreferably is at least 0.5 times, more preferably at least 1 or 2 times,most preferably at least 3, 4, 5 times or more, the total volume of theflow-through passages of the chromatography column.

In another embodiment also applicable to the chromatographic columnembodiment, the delay conduit includes a delay housing having across-sectional area transverse to fluid flow at least 3, 4, 5 times,more preferably at least 8, 9, 10 times, and most preferably at least 15or 20 times or more, the cross-sectional area of tubing in the recycleconduit.

The embodiment of FIG. 4 is similar to that of FIG. 1, and so like partswith FIG. 1 will be designated with like numbers. Here, electrolyticsuppressor 70 includes a chromatography effluent flow channel 72, filledwith ion exchange packing and first and second detector effluent flowchannels 74 and 76, respectively, in electrical communication withelectrodes 80 and 82, respectively. Ion exchange membranes, not shown,separate the first and second detector effluent flow channels 74 and 76from chromatography effluent flow channel 72. A conduit 84 connects theoutlet of flow channel 74 with the inlet of flow channel 76.

In this embodiment, the detector effluent flows sequentially through theanode chamber and cathode chamber of an electrolytic suppressor. Such anelectrolytic suppressor is similar to the one described in FIG. 2 ofU.S. Pat. No. 6,610,546, incorporated by reference. Preferably, theanode chamber and cathode chamber of this type of electrolyticsuppressors are not adjacent to each other. When the suppressor isoperated for anion analysis in an ion chromatography system with eluentrecycle, the detector effluent flows from the anode chamber to thecathode chamber. As the anode chamber effluent flows through theelectrochemically reducing cathode chamber, the reactive or oxidativespecies such as ozone and sodium percarbonate in the anode effluent areelectrochemically reduced to non-oxidative species. Thus, theconcentration of the reactive or oxidative species in the finalsuppressor effluent is reduced significantly. The use of this type ofsuppressor improves the performance of ion chromatography systems witheluent recycle.

FIG. 5 shows another embodiment of the present invention. Like partswith FIG. 4 with be designated with like numbers. In this embodiment, anenergy generator 90 is disposed between the outlet of suppressor 70 andcatalytic gas elimination column 31. As illustrated, it is disposedbetween suppressor flow channel 76 and the inlet of delay conduit 46. Insome applications, the delay conduit 46 can be eliminated and so flowwill go directly to column 31. The energy can be irradiation, as by UVlight, heat or some other energy source. For irradiation, the energygenerator may be a reaction coil including a UV light source, such as ahigh pressure mercury lamp since the decomposition of ozone in theaqueous solution is known to be accelerated by using ultravioletirradiation. The energy source may also be heat to accelerate thedecomposition of ozone in the suppressor regenerant channel effluent.The half-life of ozone in the aqueous solution (pH 7) is known todecrease from about 20 minutes to about 8 minutes when the temperatureis increased from 20° C. to 35° C.

FIG. 5 also illustrates a gas bubble detector 92 downstream of catalyticgas elimination column 31 which can be used in any of the systemsdescribed. A number of detection methods including optical andelectrical measurement techniques may be applied to detect the presenceof gas bubbles in a flowing liquid stream. The function of the gasbubble detector is to ensure the safe operation of the system in casethe catalytic gas elimination column malfunctions. If the catalytic gaselimination column malfunctions and fails to recombine hydrogen andoxygen gas in the suppressor effluent, hydrogen gas may accumulate inthe eluent reservoir to a level that present a explosion risk. Tominimize or eliminate this risk, this embodiment uses a gas bubbledetector to monitor if the eluent recycled back to the eluent reservoircontains gas bubbles. If an excessive volume of gas in the recycledeluent is detected, the gas bubble detector can be designed to provide asignal to power down the pump and turn off the electric current to theelectrolytic suppressor in an ion chromatography system with eluentrecycle to ensure its safe operation.

It is also possible to recycle ion chromatography eluent containing anorganic solvent as long as the solvent is electrochemically stable.

In another embodiment, as shown in FIG. 6, delay conduit 46 andcatalytic gas elimination column 31 can be combined in a single columndevice or housing. For example, referring to FIG. 6, column 94 includesan upstream zone or compartment 94 a which is part of the delay conduit.Compartment 94 a may be filled with packing, e.g. inert micro pellets orbeads, or may take one of the other forms of delay conduit discussedabove. The downstream, compartment 94 b (the upper compartment asillustrated) is the catalyst zone, e.g. of the type described for column31, in fluid communication with the column outlet.

Referring to FIG. 7, an ion-reflux based chromatography system usingwater recycle is illustrated using the principles and the same system asFIG. 2 of U.S. Pat. No. 7,329,346 with the addition of a delay conduit46 as described herein. The description of FIG. 2 of the '346 patent isincorporated by reference. Like parts with FIG. 1 herein will bedesignated with like numbers in FIG. 7.

The system of FIG. 7 illustrates the combined use of water purificationcolumns and the catalytic gas elimination column for recycling water inan ion-reflux based ion chromatography system that generates andrecycles potassium hydroxide eluents for anion analysis. In this ionchromatography system, the deionized water is used as the preferredcarrier stream in the electrolytic generation and recycle of potassiumhydroxide eluent. The effluent from the outlet of the eluent generationand recycle module is a mixture of water, hydrogen gas, oxygen gas, andpossibly some trace components derived from the operations of the entireion-reflux based ion chromatography system. To recycle the water, theeffluent from the eluent generation and recycle module is first passedthrough the catalytic gas elimination column to eliminate hydrogen andoxygen gases. Since stoichiometric amounts of hydrogen and oxygen gasesare generated in the electrochemical processes occurring in theelectrolytic eluent generation and recycle module and the electrolyticsuppressors, it is expected that hydrogen and oxygen combine to formwater in the same amount that is originally consumed. The effluent fromthe catalytic gas elimination column is then passed through the waterpurification column to remove the remaining trace ionic and nonioniccontaminants. The purified water is routed back to the water reservoir.

Referring specifically to FIG. 7, the solution leaving the eluentgenerator 251 in line 265 flows through catalytic gas elimination column31, through an eluent purification column 32 through line 34 intocontainer 38 for eluent reservoir 18. The recycled solution in line 34is mixed in eluent reservoir 18 and is directed via pump 20 in line 42through a second eluent purification column 44. A vent port in reservoir18 is not illustrated because hydrogen and oxygen gases may beeliminated in column 31. From there on, the system is as describedabove.

Referring again to FIG. 7, an ion chromatography system is illustratedusing a continuous electrolytically regenerated packed bed suppressor(CERPBS) form of suppressor and one embodiment of the eluent generator.The system includes an analytical pump 20 connected by tubing 212 tosample injection valve 214 which in turn is connected by tubing 216 to aflow-through chromatographic separator 218 typically in the form of achromatographic column packed with chromatographic resin particles. Theeffluent from chromatographic column 218 flows through tubing 220 to apacked ion exchange resin bed flow-through suppressor 222. Typically,suppressor 222 is formed of a column 224 packed with an ion exchangeresin bed 226 of the type used for ion chromatography suppression.Electrodes are spaced apart in the suppressor, with at least oneelectrode separated from the resin by a barrier described below. Theelectrodes are connected to a direct current power supply, not shown.The configuration is such that with an aqueous stream flowing throughthe suppressor and the application of power, water in the aqueous streamis electrolyzed to form a source of hydronium ion or hydroxide ion tocontinuously regenerate the ion exchange resin bed during the analysis.

The suppressor effluent is directed through tubing 230 to a suitabledetector 232 and then eventually to waste. A preferred detector is aconductivity detector with a flow-through conductivity cell. Thechromatography effluent flows through the cell.

Suppressor 222 generates hydronium ions (and oxygen gas) at the anodeand hydroxide ions (and hydrogen gas) at the cathode. That is, awater-containing eluent solution including electrolyte is directed fromthe pump and through tubing 212. Sample is injected through sampleinjection valve 214, and is directed by tubing 216 into chromatographiccolumn 218 to form a first chromatography effluent including separatedionic species of the sample. For simplicity of description, unlessotherwise specified the system will be described with respect to theanalysis of anions using an eluent solution including sodium hydroxideas the electrolyte.

A suitable sample is supplied through sample injection valve 214 whichis carried in a solution of eluent supplied from pump 20. Anode 236 isdisposed at the outlet end of resin bed 226 in intimate contact with theresin therein. The effluent from bed 226 is directed to a detectorsuitably in the form of a flow-through conductivity cell 232 of theconductivity detector (not shown), for detecting the resolved anions inthe effluent, connected to a conductivity meter.

The system also includes an optional component for pressurizing theeffluent from suppressor 222 prior to detection to minimize adverseeffect of gases (hydrogen or oxygen) generated in the system as will bedescribed hereinafter. Such pressurizing means comprises a pressurerestrictor 238 downstream of conductivity cell 232 to maintain the ionchromatography system under pressure.

Column 224 is typically formed of plastic conventionally used for an ionexchange column. It has a cylindrical cavity of a suitable length, e.g.,60 mm long and 4 mm in diameter. It is packed with a high capacitycation exchange resin, e.g., of the sulfonated polystyrene type. Theresin is suitably contained in the column by a porous frit which servesto provide an outlet to the column. In the illustrated embodiment, theporous frit is porous electrode 236 which serves the dual function ofcontainment of the resin and as an electrode.

A barrier 240 separates bed 226 from electrode 242 in the interior of ahollow housing defining an ion receiving flow channel in electrodechamber 244 preventing any significant liquid flow but permittingtransport of ions only of the same charge as the charge of exchangeableions on resin bed 226. For anion analysis, barrier 240 is suitably inthe form of a cation exchange membrane or plug separating electrodechamber 244 from the cation exchange resin.

A conduit 248 is provided to direct the aqueous liquid stream to theinlet 250 of electrode chamber 244. Conduit 252 takes the effluent fromchamber 244 to the eluent generator 251. This provides a means of makingelectrical contact with the electrodes that is at the same time easy toseal against liquid leakage.

The line X-X is illustrated across the resin bed 226. For reasons whichwill be explained below, the resin upstream of the dotted line may bepredominantly or completely in the form of the cation counter ion of thebase used as the electrolyte during separation. Downstream of the lineX-X, the resin may be predominantly or completely in the hydronium form.The line X-X represents the interface.

For anion analysis, a polarizing DC potential is applied between cathode242 and anode 236, and the following reactions take place.

The water is electrolyzed and hydronium ions are generated at anode 236according to the following reaction:

H₂O-2e ⁻→2H⁺+½O₂↑

This causes cations in the cation exchange resin bed 226 to migrate tobarrier 240. This, in turn, displaces hydronium ions upwardly throughbed 226 which causes a similar displacement of cations ahead of them.The cations electromigrate toward the barrier 240 to be transportedacross the barrier 240 toward cathode 242 in cathode chamber 244 whilewater is electrolyzed at cathode 242 to generate hydroxide ionsaccording to the following reaction:

2H₂O+2e⁻→2OH⁻+H₂↑

The cations which have transported across the barrier combine with thegenerated hydroxide ions to form cation hydroxide in cathode chamber244. The effluent from separator bed percolates through the cation formresin in inlet bed section 226 until it reaches the hydronium form resinin bed section 226 where it is neutralized while the cation is retainedon the resin. At this point, the anion salts are converted to theirrespective acids and the cation hydroxide is converted to weakly ionizedform, water.

The suppressed effluent liquid containing the separated anions leavesbed 226 through conduit 230 and passes to conductivity cell 232 in whichthe conductivity of the separated anions is detected.

The suppressor of FIG. 7 has been described with respect to a system forthe analysis of anions. However, the system is also applicable to theanalysis of cations. In this instance, electrode 236 is a cathode andelectrode 242 is an anode. The ion exchange type of resin is reversed.Thus, the resin in separator bed 218 is a cation exchange resin and theresin in suppressor bed 226 is an anion exchange resin. The plug ormembrane 240 is made of an anion exchange material.

Briefly described, the suppressor works as follows for the cationanalysis. The aqueous liquid stream containing cations to be detectedand an acid electrolyte aqueous eluent are directed through separatorbed 218 including cation exchange resin. The effluent from separator bed218 flows through suppressor bed 226 including anion exchange resin withexchangeable hydroxide ions. The acid in the eluent is converted toweakly ionized form. Some of the exchangeable hydroxide is displaced byanions from the acid.

An electrical potential is applied between the cathode 236 and anode242. Water is electrolyzed at electrode 236 to generate hydroxide tocause anions on the anion exchange resin bed to electromigrate towardbarrier 240 to be transported across the barrier toward the positivelycharged anode 242 in the ion receiving flow channel in electrode chamber244 while water in chamber 244 is electrolyzed to generate hydroniumions which combine with the transported anions to form acid in theelectrode chamber 244. The effluent liquid from the suppressor bed 226flows past detector 232 in which separated cations are detected and isrecycled to electrode chamber 244.

Referring again to FIG. 7, one embodiment of the eluent generator 251 isillustrated, describing first the system for anion analysis in which abase generated in electrode chamber 244 is directed to the eluentgenerator 251. This embodiment is analogous in electrochemical operationto suppressor 224. In this embodiment of the eluent generator, asuitable housing 254 contains an electrolyte ion reservoir in the formof a packed bed of ion exchange resin 256. Resin bed 256 is separatedfrom a first generator electrode chamber 258 by a charged generatorbarrier 260 which prevents significant liquid flow but permits transportof electrolyte ions and thus may be of the type described with respectto suppressor barrier 240. A generator electrode 262 is disposed andenclosed in generator electrode chamber 258 and may be of the same typeof construction as electrode chamber 244. At the opposite side ofbarrier 260 from electrode 262 is flow-through generator electrode 264analogous in function and structure to suppressor electrode 236.

The electrochemical reactions described above with respect to thesuppressor occur in the eluent generator and so are incorporated hereinby reference. Thus, for analysis of anions, the line x-x separates theinlet section 256 a from the outlet section 256 b of resin bed 256. Thefeed stream in line 252 flows into inlet section 256 a in the cationform while the outlet section is in the hydronium ion form. However, onedifference is that the feed stream in conduit 252 already includes base.The feed stream exits packed resin bed 256 adjacent barrier 260 andflows across bed 256 and out the outlet through electrode 264 or pastsome other form of electrode as described above. Similarly, the packedbed includes resin in the electrolyte ion form (e.g., potassium orsodium) at its inlet end adjacent barrier 260 and in hydrogen ion formnear the outlet end adjacent electrode 264.

The same type of packed bed resin or other form of matrix may be used inthe eluent generator as in the suppressor. As illustrated, the source ofaqueous liquid flowing through generator electrode chamber 258 can beliquid recycled from the outlet of the resin bed chamber 254.Specifically, such liquid flows through conduit 265 and into a mixed(cation and anion exchange) bed water polishing column, or eluentpurification column 32. This column is typically 2 to 40 cm in lengthand 0.5 to 10 cm in internal diameter. From there, the stream flowsthrough conduit and flows to eluent reservoir 18. In the case of anionanalysis, the cation hydroxide is generated in chamber 258 adjacent thecathode in the manner described above with respect to the suppressor.The water source after passing through the water polisher is deionizedand so does not interfere with the analysis. An optional eluentpurification column 44 may be placed after pump 20 to further purify thedeionized water stream

Delay conduit 46 is disposed between suppressor 224 and eluent generator251. An advantage of this location is that the unstable oxidativecompounds that could be detrimental to the catalyst and/or ion exchangemedium in column 31 could also be detrimental to the ion exchange mediumin eluent generator 251. Thus, the delay in conduit 46 can reduce theharmful effect of such compounds. However, the delay conduit can bedisposed in other locations such as between cell 232 and column 31,preferably between suppressor 224 and column 31.

It is useful to discuss the principles of operation of a catalytic gaselimination chamber typically contained in a flow-through housing termeda column herein. Even though the combination of hydrogen and oxygen toform water is an exothermic process, hydrogen and oxygen do not reactautomatically when mixed together. The reason for this is the relativelylarge activation energy needed to begin the reaction. The mechanism issomewhat complex. It is a free radical mechanism with one of theinitiation steps is:

H₂(g)→2H.(g)

Breaking the bond between the two hydrogen atoms requires 432 kJ/mole.This energy can be initially provided by a spark or a flame. After thereaction begins, the produced energy provides the necessary energy tocontinue breaking apart the hydrogen molecules. A catalyst provides analternative mechanism that has a lower activation energy, this allowsthe reaction to proceed without the requirement of the initial additionof energy such as a flame or spark.

According to the invention, preferred metal catalysts in catalytic gaselimination column 31 include one or more metals from the Platinumgroup. Such Platinum group metals are defined herein to include, inorder of increasing atomic weights, ruthenium, rhodium, palladium,osmium, iridium and platinum. Such metal catalysts could be usedseparately or in combination or could be incorporated in an alloy, forexample, a platinum nickel alloy. The invention will first be describedwith respect to platinum metal catalysts.

Platinum is known to catalyze the reaction of hydrogen and oxygen.(Nature, vol. 390, 495-497, 1997; Journal of Chemical Physics, vol. 107,6443-6447, 1997; Surface Science, vol. 324, 185-201, 1995.) If platinumis placed into a container filled with hydrogen and oxygen, the platinumbegins to glow as it heats up, and water droplets condense in thecontainer. Cooling the platinum so that it doesn't just ignite themixture provides a smooth conversion of the hydrogen and oxygen towater. This reaction occurs because platinum provide a new route for thereaction. In this new pathway, hydrogen molecules react with theplatinum atoms on the surface of platinum. This reaction breaks the H—Hbond and forms two Pt—H bonds. The energy of activation for this processis small. Oxygen reacts with these Pt—H groups to form water, again,with a very small energy of activation. Through this low energy routefor the reaction, platinum catalyzes the recombination of oxygen andhydrogen gases. The term “catalyst” encompasses any catalyst thatperforms this function. Platinum is described as one specific example ofsuch a catalyst.

The above principles of using the known property of a catalyst such asplatinum to catalytically induce the reaction between hydrogen gas andoxygen gas to form water provide the basis for the catalytic gaselimination chamber of the present invention for use in liquidchromatography. This permits the elimination of these gaseouselectrolysis byproducts of an electrolytic suppressor in the eluent in asystem like FIG. 1. In such a system, the effluent from the outlet ofthe electrolytic suppressor regenerant chamber 24 is passed through thecatalytic gas elimination column 31, where the hydrogen and oxygenpresent in the effluent react catalytically to form water. Column 31 maybe packed with a suitable catalyst such as pure Pt in metal particle,mesh, or foil form. Alternatively, the column may also be packed withother inert substrates coated with Pt. In the present invention,optional catalytic gas elimination column 31 serves several importantfunctions. First, it provides an elegant means to conveniently eliminatethe build up hydrogen and oxygen gases and thus facilitates theoperation of continuous eluent recycle. Second, the water-formingreaction of hydrogen and oxygen is expected to be stoichiometric in thecolumn, and the amount of water formed is expected to be in principlethe same as the amount of water that is originally consumed to producehydrogen and oxygen gases in the electrolytic operation of thesuppressor. In principle, this feature eliminates an increase in theconcentration of eluent due to the consumption of water in theelectrolytic operation of the suppressor. Third, the Pt catalytic gaselimination column also serves the function to catalytically decomposetrace levels of hydrogen peroxide which may be formed during theelectrolytic operation of the suppressor. The presence of hydrogenperoxide in the recycled eluent is potentially detrimental since it mayattack the ion exchange functional groups in the separation column andthe eluent purification column in the system and degrade the columnperformance. The use of Pt to catalyze the decomposition of hydrogenperoxide is well known (Bull. Korean Chem. Society, 1999, vol. 20(6),696; U.S. Pat. No. 6,228,333).

The catalytic gas elimination chamber may have a wide variety ofphysical forms. One embodiment uses a chromatographic column housingwith fritted flow-through end fittings that are used to retain eitherpure Pt metal particles, mesh, or foils packed inside the column. Thecolumn may also be packed with other inert substrates coated with Pt. Ina preferred embodiment, the internal diameter of the column is 0.1 mm orlarger and the length of the column is 0.5 cm or longer. It is preferredto use Pt packing material in forms that provide high surface area toincrease its catalytic efficiency. It is also preferred to operate thecatalytic gas elimination chamber in flow rates ranging from 0.1 uL/minto 50 mL/min although other flow rates may be used.

Another embodiment relates to a catalytic gas and ionic species removaldevice (“the CGISRD”). It is particularly useful in a chromatographysystem, particularly the ion chromatography systems disclosed herein. Ingeneral, the device includes a liquid flow-through housing, a Platinumgroup metal catalyst for catalytically combining hydrogen and oxygengases, or for catalytically decomposing hydrogen peroxide, or both,disposed in a housing, and flow-through ion exchange medium alsodisposed in the housing. In preferred embodiments, the Platinum groupmetal catalysts are platinum, palladium, or mixtures thereof, in solemetal or alloy form. As set out herein, the Platinum group metalcatalysts may also include ruthenium, osmium, rhomium, and iridium, insubstantially pure metals form or in an alloy form.

The CGISRD can be formed as a flow-through housing of the type describedherein regarding catalytic gas elimination column 31 in FIG. 1. However,in addition to the Platinum group metal catalyst, the CGISRD includesflow-through ion exchange medium disposed in the housing. Suitableflow-through ion exchange medium may include an ion exchange particulatebed or an ion exchange monolith, such as described in U.S. Pat. No.7,074,331. In some embodiments, the metal catalyst is not bound to theion exchange medium. Here, the ion exchange medium can be disposedupstream or downstream of the metal catalyst or can be mixed with themetal catalyst. An advantage of this approach is that it eliminates theneed for having a separate column containing the ion exchange phase forremoving analyte ions, counterions or both and having a separate columnfor the metal catalyst, thus lowering the overall cost of the hardwarerequired for this work. Additionally having a single column enclosureminimizes the number of fittings and tubing connections.

In a preferred embodiment of the CGISRD, the Platinum group metalcatalysts is irreversibly bound as a coating to said ion exchangemedium, as by electrostatic binding. The device has sufficient ionexchange capacity for ion exchange with ions, such as sample ions orsample counterions, in a flowing liquid stream so that a substantialamount of, preferably most or all of, such ions are bound for removalfrom the flowing liquid prior to recycle to the separation medium as isperformed in eluent purification column 32 of FIG. 1 to the ion exchangemedium. Such ion exchange capacity may be partially or completelyprovided by the coated ion exchange medium by providing excess ionexchange capacity in the medium which is not consumed by irreversiblebinding to the coating. Thus, when removal of such ions is performed bythe coated ion exchange medium, the excess capacity of the ion exchangephase of the catalyst coated ion exchange material that is available forthe removal is preferably more than 0.1 times, more preferably 0.3 timesand most preferably 0.5 times or more of the capacity of the uncoatedion exchange phase. Such ion exchange capacity can be supplemented byadding uncoated ion exchange medium to the coated ion exchange medium inthe device. This added ion exchange capacity would aid improved removalof sample ions or sample counter ions.

In one embodiment, the metal catalyst is bound to the ion exchangemedium by using the metal in a reagent which includes ionic moietieswhich irreversibly bind electrostatically to the ion exchange moietieson the ion exchange medium. Suitable metal catalyst reagents forattaching to ion exchange resins are metal compounds, preferablymetal-containing amine cations for attaching to the cation exchangeresins or metal-containing chloro-compounds that are metal-containinganions for attaching to anion exchange resins. The reagents preferablywould be electrostatically bound to the ion exchange moieties beforethey are permanently attached to the surface. Such reagents includetetraammineplatinum (II) chloride, diamminepalladium (II) nitrite,sodium hexachloro palladate (IV), sodium hexachloro platinate (IV).ammonium pentachloroaquo rhodate (III), potassium pentachlororhodate(III), pentamminechlororhodium(III)dichloride, ammonium pentachloroaquoruthenate (III), hexaammine ruthenium (III) chloride, ammoniumhexachloro iridate, (III), potassium hexachloro iridate (IV), potassiumhexachloro osmate (IV) and the like. Such reagents could be usedseparately or in mixtures for coating various catalyst metals on top ofthe ion exchange materials. For example, two or more reagents could bemixed and electrostatically bound to the cation exchange resin phase invarying ratios. This would yield phases with a mixed catalyst containingsurface. For example, the combination of platinum and palladium in acoated phase yields excellent catalytic decomposition of hydrogenperoxide in addition to good catalytic formation of water from theelectrolytic gases. Thus, the catalyst coated materials could betailored for a given application. In one coated embodiment, the metalcatalyst is in a thin layer, preferably a monolayer, to minimize thetotal catalyst used. Also, after synthesis, it is possible to use amixture of various catalyst coated ion exchange materials.

In a preferred embodiment, the Platinum group metal catalyst comprisesboth platinum and palladium. For example, the ion exchange resin couldbe coated individually in two separate synthesis steps with platinum andpalladium, respectively, and then the resin phase is mixed together andpacked into a column. This would yield a combination of a platinum andpalladium coated phase. It should be noted that the two phases in theabove embodiment could also be packed in separate layers in a two layeror multi layer embodiment without substantially mixing the two phases.In another embodiment the two metal catalyst reagents are intimatelymixed together and are attached to the ion exchange resin in onesynthesis step. This would yield a surface that would have both reagentsand ultimately both metal catalysts on the surface of the ion exchangeresins. Using some resin phases, it is also possible to coat the resinwith one catalyst metal followed by another coating step with anothercatalyst metal.

For anion analysis, the preferred ion exchange medium is in the cationexchange form. The residual cation exchange capacity can be used forremoving the counter ion cation to the analyte ion. For example whenanalyzing drinking water the counter ions are typically monovalent anddivalent cations such as sodium, ammonium, calcium and magnesium. Forcation analysis the preferred ion exchange medium is in anionic form toremove anion counterions to the sample cations.

As set forth above, the catalyst coated ion exchange medium can be mixedwith uncoated substrate ion exchange medium for enhancing the retentionof counterions. In applications where it is desired to remove bothcounterions and analyte ions, such as after electroelution separation ofanalyte ions discussed hereinafter, a mixture of the catalyst coated ionexchange medium and ion exchange medium of opposite charge as in ormixed bed of both charges may be used.

Suitable ion exchange materials are disclosed in the prior art and arecommercially available from Dow Chemical Co. as cation exchange or anionexchange materials sold as Dowex Ion exchange resins, or from Rohm andHaas as Amberjet or Amberlite and the like. The ion exchange materialshave ion exchange groups that could be modified as per the presentinvention with a catalyst metal coating.

It should also be noted that the synthesis of the disclosed materialscan be done in bulk before packing it into columns or could be doneinsitu within columns. For cost and practical reasons the former ispreferred over the latter.

The systems of all embodiments are also applicable to the generation ofa base eluent with appropriate reversal of polarity of the reagents andcharged components for anion analysis.

FIG. 8 illustrates a chromatography system using a catalytic gaselimination column and an electro elution separator column. An aqueousstream source container 110 with an aqueous source 112 is connectedfluidically to a pump 116, which is connected to an injection valve 120by conduit 118. This portion of the plumbing is similar to ionchromatography systems of the prior art. The injection valve is thenplumbed to a chromatography separator 122 as in the prior art. Theseparator column 122 is packed with appropriate separator medium (notshown). Two flow-through electrodes 128 and 130 flank the ends theseparation medium. The electrodes are connected to a power supply (notshown) via connectors 124 and 126. The fluidic line out of column 122 isconnected to the inlet of catalytic gas elimination column 134 which maybe built and operated as described for any of the embodiments herein.The outlet of catalytic gas elimination column is connected to adetector cell 138 (preferably a conductivity cell). The outlet of thecell 138 is diverted to waste 140 or routed back to the source container110.

In operation, an aqueous stream, which may be water without anelectrolyte, is pumped from container 110 and is routed via conduit 118to injection valve 120. The stream is then routed out of valve 120 withor without the injected sample into separation column 122 for separatingthe individual components. Column 122 produces both hydrogen and oxygengases are catalytically recombined back to water in catalytic gaselimination column 134 as described for any of the embodiments herein.In addition any peroxide is also decomposed according to the presentinvention. For the embodiment of FIG. 1 herein, in which ion exchangemedium is not used in catalytic gas elimination column, the analytecounter ions that are unretained in the column 122 may be removed in apurification column such as described for column 32 in FIG. 1. Forcatalyst column 134, which includes ion exchange medium as describedherein, such ions may be removed in column 134. The sample ions arerouted from catalyst column 134 and are substantially unimpeded into thedetector cell 138 for detection. The aqueous stream is routed to waste140 or could be recycled using conduit 142 back into the aqueous sourcestream container 110. One or more trap columns (not shown) could beinstalled in line 142 to remove the analyte ions or any neutral species.

Construction and operation of electro elution separator 122 may beaccomplished as described in U.S. Pat. No. 6,093,327, particularlycolumns 11-18, incorporated herein by reference. In this system, theeluent can be an aqueous liquid such as deionized water, or an acid,base, or salt-containing aqueous solution.

In another embodiment (not shown), catalytic gas elimination column 134could be installed after the detector cell 38. In this embodiment somebackpressure is preferred to compress the electrolytic gases to aiddetection. Column 134 in this embodiment would allow the catalysisreaction, coupled with aiding the decomposition of peroxide and inaddition aiding the removal of analyte ions or counterions or both. Inthis embodiment the aqueous stream is routed via conduit 142 back intothe aqueous source stream container 110. A trap column (not shown) couldbe installed in line 142 to remove any neutral species. In addition apurifier column could be used to purify the aqueous stream in line 114or line 118.

In the above embodiments of the present invention, water formed by thecatalytic combination of hydrogen and oxygen gas can serve as a sourceof water for the eluent stream by flowing the produced water into theeluent stream. The gases can be electrolytically generated in thechromatography system or supplied from independent sources of hydrogenand oxygen gas such as from pressurized containers.

The following examples demonstrate the present invention for systems inwhich the eluents are prepared off-line and recycled and water is usedin the electrolytic eluent preparation in ion chromatography.

Example 1

Ion chromatographic separation of common anions using an electrolyticsuppressor and recycle of a sodium carbonate/sodium bicarbonate eluent.

This example illustrates the use of the eluent-recycle ionchromatography system shown in FIG. 1 for determination of common anionsincluding fluoride, chloride, nitrate, phosphate and sulfate. A DionexICS-2000 ion chromatography system consisting of a dual-piston highpressure pump, a six-port injector, a column oven, and a conductivitydetector was used. A Dionex 4-mm AS22 column (4 mm×250 mm) was used asthe separation column. A solution of 4.5 mM sodium carbonate and 1.2 mMsodium bicarbonate was used as the eluent, and the separation wasperformed at 1.2 mL/min. A Dionex ASRS-300 electrolytic suppressor wasused in the experiments. A flow-through delay column (10 mm×250 mm) wasused. The catalytic gas elimination column contains a cation exchangeresin coated with Pt. The eluent purification columns (9 mm×85 mm) werepacked with appropriate ion exchange resins. The analyte trap column (9mm×50 mm) was packed with an aminated anion exchange resin.

In one set of experiments, sample solutions containing common anionssuch as fluoride, chloride, bromide, nitrate, phosphate, and sulfatewere injected daily, the retention time of each analyte were monitoredover a period of 400 hours during which a 4-liter solution of 4.5 mMsodium carbonate and 1.2 mM sodium bicarbonate was recycledcontinuously. If the eluent was not recycled, the total eluentconsumption would have been 28.8 liters over the period of 400 hours.FIG. 9 shows the retention time reproducibility data obtained for thetarget analytes. The retention time percent RSD for the target analytesranged from 0.48% for phosphate to 1.3% for sulfate over the period of400 hours. These results indicate that the ion chromatography systemshown in FIG. 1 can be used to perform reproducible separation ofanalyte ions of interest using recycled sodium carbonate eluent over anextended period of time. The operation of an ion chromatography systemin such a format simplifies the system operations, minimize wastedisposal, and reduce operating costs.

Example 2

Ion chromatographic separation of common cations using an electrolyticsuppressor and recycle of a methanesulfonic eluent.

This example illustrates the use of an ion chromatography system witheluent recycle shown in FIG. 1 for determination of common cationsincluding lithium, sodium, ammonium, potassium, magnesium, and calcium.A Dionex ICS-2000 ion chromatography system consisting of a dual-pistonhigh pressure pump, a six-port injector, a column oven, and aconductivity detector was used. A Dionex 4-mm CS12A column (4 mm×250 mm)was used as the separation column, a solution of 20 mN methanesulfonicacid was used as the eluent, and the separation was performed at 1.0mL/min. A Dionex CSRS-300 electrolytic suppressor was used in theexperiments. A flow-through delay column (10 mm×250 mm) was used. Thecatalytic gas elimination column contains a cation exchange resin coatedwith Pt. The eluent purification columns (9 mm×85 mm) were packed withappropriate ion exchange resins. The analyte trap column (9 mm×50 mm)was packed with a fully sulfonated cation exchange resin.

In one set of experiments, sample solutions containing lithium, sodium,ammonium, potassium, magnesium, and calcium was injected daily, theretention time of each analyte were monitored. FIG. 10 shows therepresentative separations of six cations over the period of 60 days.These results indicate that the ion chromatography system shown in FIG.1 can be used to perform reproducible separation of cations of interestover an extended period of time. The operation of an ion chromatographysystem in such a format simplifies the system operations, minimize wastedisposal, and reduce operating costs.

Example 3

This Example describes a process of preparing a platinum coated resinfor this application. A cation exchange resin (commercially availablefrom various resin manufacturers such as Rohm and Haas; Dow Chemicalsetc) was used as the substrate. 100 g of the resin was converted to thesodium form using 1 M sodium hydroxide. Next the resin was washed withDI water and filtered. The resin was added to a 2 L bottle and theappropriate catalyst solution (For example Tetraammineplatinum (II)Chloride solution) app. 630 ml at a concentration of 4000 mg/L wasadded. The bottle was capped and tumbled to mix the resin intimatelywith the catalyst solution for 2 hours. The resin was washed andfiltered. At this point a layer of platinum is adhered to the resin byelectrostatic means. Next, a 5% borohydride solution was made and theresin was placed in this container and mixed with care. The container isleft open to ambient environment (due to bubbling in the container) andplaced in an oven at 65° C. for 4 hours. After 4 hours the resin wasfiltered and washed with DI water for four times to remove all thereactants. The resin appears blackened and is now ready for catalyticoperation.

Example 4

A 10 um resin (Ethylvinylbenzene 55% cross linked with divinylbenzene)was used as the substrate in this experiment. The resin was coated witha) 4000 mg/L Tetraammineplatinum (II) Chloride solution 2) 4000 mg/LDiamminepalladium (II) nitrite solution and 3) A combination of 1) and2) at a concentration of 2000 mg/L each. The resin synthesis protocolfollowed the outline listed above in example 3. Three standardchromatography guard columns from Dionex Corporation with a 9×50 mmdimensions were packed with the above coated resins using DI water asthe packing media.

The columns were tested for peroxide removal using a DX600 IC systemwith an electrochemical detector from Dionex Corporation, Sunnyvale,Calif. A CP PA20 column was used with an eluent comprising 100 mM NaOHat 0.5 ml/min flow rate and at 30° C. A gold electrode was used as theelectrode and detection was pursued with a pre-loaded quadruple waveformwith AgCl reference electrodes. A 10 μL injection sample loop was usedin this work. For testing the catalytic function an optical sensor wasused to detect the transition of the bubble. A voltage response to thetransition of the bubble was recorded. It should be noted that thissetup would detect all bubbles and any bubbles originating from leakingfittings etc would also be detected. Therefore care was exerted to keepall the fittings finger tight and leak free. The voltage signal from thebubble detector was fed to a UI 20 interface module from DionexCorporation and the signals were collected using Chromeleon®, aChromatography software from Dionex Corporation (Sunnyvale, Calif.). Inthe experimental setup a 4 mm ASRS suppressor from Dionex Corporationwas pumped with 9 mM sodium carbonate eluent at a flow rate of 1 ml/minand the suppressor waste was diverted into a 9×50 mm catalyst column asper the present invention and the fluid eluting out was diverted into an⅛″ Teflon tubing across on which was mounted an optical sensor fordetection of bubbles. This setup as per the present invention wassufficient to monitor the transition of bubbles. The fluid eluting outof the ⅛″Teflon tubing was routed to a container or was directly routedto an injection valve for peroxide detection. In some cases a knownstandard amount of peroxide was pumped into the catalyst column and wasrouted to the injection valve for testing the peroxide removalefficiency.

Example 5

FIG. 11 shows the peroxide removal efficiency of the devices. A 10 mg/Lperoxide standard was used in this experiment and tested as per thesetup discussed in example 4. Trace A in FIG. 11 showed the performanceof a platinum coated ion exchange resin; trace B showed the performanceof a palladium coated ion exchange resin and trace C showed theperformance of a combination (palladium and platinum) coated resin. Itis clear from the intensity of peak 1 that most of the peroxide wasremoved by the setup of trace C. All three traces showed significantremoval of peroxide as apparent from the intensity of peak 1. The peakarea for a standard run (not shown) was 4.084 and the three catalystcolumns showed good removal efficiency for peroxide as shown in Table 1.Here the removal efficiency is expressed as a % of the control which hasno catalyst resin.

TABLE 1 Peroxide removal results with a 10 um resin (Ethylvinylbenzenebased and 55% cross linked with divinylbenzene) Catalyst Coating PeakArea Removal Efficiency % None 4.084 Palladium 0.0063 99.85 Platinum0.0139 99.66 Palladium + Platinum 0.0012 99.97

Example 6

FIG. 12 shows bubble removal comparison between the three testedcolumns. Bubbles here are detected as negative spikes of the voltagesignal. The more the number of bubbles detected the poorer the catalystfunction. Trace A, B & C display the performance of the three catalystresins as discussed previously in Example 5. Excellent bubble removal isseen with a combination of Platinum and Palladium coated resin.

Example 7

Since the bubbles were not completely removed we investigated anotherresin for this application. This is a polystyrene divinylbenzene basedproprietary resin from Dionex Corporation (Sunnyvale, Calif.) that was2% cross-linked and had a diameter of 15.4 micron. This resin was coatedwith the three combinations as discussed above following the protocoldescribed in Example 3. The peroxide removal is shown in FIG. 13 andshows once again the combination (trace C) outperforming the singlemetal coated resins. The results are summarized in Table 2 below.

TABLE 2 Peroxide removal results with a 15.4 um resin (Styrene based and2% cross linked with divinylbenzene) Catalyst Coating Peak Area RemovalEfficiency % None 5.03 Palladium 0.0058 99.88 Platinum 0.0135 99.73Palladium + Platinum 0.0016 99.97

Example 8

The catalytic gas removal was tested for the ion exchange substrate ofExample 7 (trace A, B & C of FIG. 14) and showed excellent gas removalfor the combination metal catalyst as shown in Trace C.

Example 9

A 50 um resin (Polystyrene 16% cross linked with divinylbenzene) fullysulfonated resin from Dionex Corporation was used as the substrate inthis experiment. All other synthesis conditions were similar to Example3 and the test setup was similar to Example 4. Referring to FIG. 15, theperoxide removal results showed the palladium (B) to outperform theplatinum (A) and the combination (C) outperformed both. Complete removalwas possible by the combination as apparent from a peak close to S/Nratios. Table 3 summarizes the results below. Also the catalytic gasremoval efficiency was tested and showed platinum (A) to outperformpalladium (B) and the combination coatings (C) with the two metalsshowed no bubbles and outperformed the single metal coatings (FIG. 16).

TABLE 3 A 50 um resin (Polystyrene 16% cross linked with divinylbenzene)fully sulfonated resin Catalyst Coating Peak Area Removal Efficiency %None 3.69 Palladium 0.0045 99.88 Platinum 0.0163 99.56 Palladium +Platinum 0.0002 99.99

What is claimed is:
 1. A chromatography apparatus comprising: (a) anelectro elution separator including a chromatographic separation mediumdisposed in a column lumen, and spaced electrodes in electricalcommunication with the chromatographic separation medium and disposed topass an electric current through the chromatographic separation medium;(b) a catalytic gas elimination chamber in fluid communication with theelectro elution separator, the catalytic gas elimination chamber beingdownstream of the electro elution separator, the catalytic gaselimination chamber including a catalyst for combining hydrogen andoxygen gases, or for catalytically decomposing hydrogen peroxide, orboth, in an effluent stream to form water and reduce the gas content inthe eluent stream; and (c) a detector in fluid communication with theelectro elution separator and the catalytic gas elimination chamber, thedetector being downstream of the electro elution separator.
 2. Thechromatography apparatus of claim 1 further comprising: (d) a firstconduit for the effluent stream providing fluid communication betweenthe electro elution separator and the catalytic gas elimination chamber.3. The chromatography apparatus of claim 2 further comprising: (e) asecond conduit for the effluent stream providing fluid communicationbetween the catalytic gas elimination chamber and the detector.
 4. Thechromatography apparatus of claim 2 further comprising: (f) a recycleconduit for recycling the effluent stream from the detector to theelectro elution separator.
 5. A chromatographic method comprising:pumping an aqueous stream into an electro elution separator including achromatographic separation medium disposed in a column lumen, and spacedelectrodes in electrical communication with the chromatographicseparation medium; injecting sample ionic species into the electroelution separator, chromatographically separating the sample ionicspecies in the aqueous stream by flowing the sample ionic speciesthrough the chromatographic separation medium, while applying anelectric current across the chromatographic separation medium togenerate a hydrogen gas and an oxygen gas, to exit as a chromatographyeffluent, catalytically combining the hydrogen gas and the oxygen gas orcatalytically decomposing hydrogen peroxide, or both, in thechromatography effluent by flowing it through a catalytic gaselimination chamber, to form water and reduce the gas content of thechromatography effluent exiting the gas elimination chamber, and flowingthe chromatography effluent through a detector to detect the separatedsample ionic species in the chromatography effluent.
 6. Thechromatographic method of claim 5 further comprising: detecting thechromatographically separated sample ionic species at the detector. 7.The chromatographic method of claim 5 further comprising: recycling thechromatography effluent from the detector to the electro elutionseparator.
 8. The chromatographic method of claim 5, in which theaqueous stream does not have an electrolyte.